1. Field of the Invention
The present invention generally relates to a solid state image pickup apparatus and more specifically to an improvement in the construction of its charge transfer device of the apparatus and a method of driving the same.
2. Description of the Prior Art
FIG. 1 is a plan view useful to explain a prior art solid state image pickup apparatus of FIT (frame interline transfer) type. In FIG. 1, there are alternately arranged in the row (from left to right) direction photosensitive element columns 1 including 8 photosensitive elements I.sub.1 to I.sub.8, for instance, and column direction transfer devices 2 for transferring signal charges of each photosensitive element column 1 in the column (from top to bottom) direction. Further, the column direction charge transfer device 2 is made up of 4 stages, each stage corresponding to 2 photosensitive elements.
On each lower side of each column direction charge transfer device 2, is provided a selective electrode 3, and thereunder a pair of two column charge storage/transfer devices 4 and 5 are arranged. This selective electrode 3 electrically connects each column direction charge transfer device 2 to one of the charge storage/transfer devices 4 and 5. A pair of charge storage/transfer devices 4 and 5 includes a total of 8 transfer stages and functions as a frame memory for storing all signal charges .circle.1 to .circle.8 of the 8 one column photosensitive elements. .In more detail, odd numbered signal charges .circle.1 , .circle.3 , .circle.5 .circle.7 (1st field) of photosensitive elements are read out into transfer stages of the column direction charge transfer device 2, and then alternately transferred to the charge storage/transfer devices 4 and 5 via the selective electrode 3. Upon completion of the first field column direction- transfer operation, even numbered signal charges .circle.2 , .circle.4 , .circle.6 and .circle.8 (2nd field) of photosensitive elements are read out into corresponding transfer stages of the column direction charge transfer device 2, and then transferred in the column direction as in the first field. FIG. 1 shows the signal charge arrangement in the charge storage/transfer devices 4 and 5 obtained when the second field column direction transfer operation has been completed. Thereafter, all signal charges .circle.1 of all columns are parallelly or simultaneously transferred to a row direction transfer device 6, and then transferred in the row direction (from right to left in FIG. 1). Thereafter the signal charges are outputted through an output circuit 7 as electric signals. The same charge transfer operation is consecutively repeated in the order of .circle.3 , .circle.5 , .circle.7 , .circle.2 , .circle.4 , .circle.6 and .circle.8 .
In the prior art solid state image pickup apparatus described above, transfer stages of the charge storage/transfer devices are formed integrally along the row direction because of a restriction on wirings. Therefore, all charge storage/transfer devices can only transfer signal charges in the same direction (from top to bottom in FIG. 1) at the same time. Therefore, the charge arrangement is limited to only the one shown in FIG. 1 when all signal charges are stored in the charge storage/transfer devices after twice column direction charge transfer operations of 1st field and 2nd field. In other words, it is impossible to output signal charges in the order of row numbers .circle.1 , .circle.2 , .circle.3 , .circle.4 , .circle.5 , .circle.6 , .circle.7 and .circle.8 . Therefore, it is difficult to process charge signals, for instance when a vertical correlation processing is required between two adjacent rows.
Further, in a apparatus in which each of transfer stages of the column direction transfer device 2 corresponds to 4 photosensitive elements arranged in the column direction, it is difficult to form one field signals suitable for television signals, because signal charges are read out in the order of .circle.1 , .circle.5 , .circle.2 , .circle.6 , .circle.3 , .circle.7 , .circle.4 and .circle.8 . In other words, it is necessary to provide at least one transfer stage for every two photosensitive elements.
In addition, since the charge transfer capacity is roughly proportional to the area of one transfer stage, it is necessary to increase the channel width (right and left direction in FIG. 1) of the column direction charge transfer devices in order to obtain a large charge transfer capacity. This causes another problem when the apparatus is required to be highly integrated.
Further, since each selective electrode must transfer signal charges to two charge storage/transfer devices, it is necessary to increase the charge transfer speed in this charge selection operation.